Bivalves Fouling Floating Cages Used in Aquaculture

Indian Journal of Marine Sciences Vol. 11, September 1982, pp. 283-285

PO J BWATHONDI & M A K NGOILE University of Oar es Salaam, Institute of Marine Sciences, P.O. Box 668, Zanzibar, Tanzania Received 20 July 1981; revised received 7 January 1982

Nine species of bivalve foulers were identified on floating cages used in aquaculture of rabbit fish, Siganus canalicullatus. Ostrea spp and Pinctada spp (5 species) were most dominant. Settlement of oysters was bi-weekly with the rate of settlement accelerated by the onset of rains.

Aquaculture of rabbit fish Siganus eanalieullatus has of individual bivalve species both on the outer and recently been introduced in Tanzania!. Settlement of inner surface (from one cage) and combined (from the fouling organisms on marine cages employed in other cage) were recorded and compared (Table 1). aquaculture is a major problem, therefore information Length measurements of 3 species, Pine tad a vulgaris, on the biology offouling organisms in Tanzania waters Pine/ad a sp. and Os/rea sp. were taken. Length of the is of importance for successful fish culture using cages. hinge from anterior to the posterior was taken as the This paper reports the settlement and growth of length of bivalves with soft shell (Pine/ada group) while bivalves which foul marine cages. This information maximum length of the shells was taken to be the may be useful in establishing ways of combating the length of the bivalves with hard shells (Os/rea group). fouling bivalves. Monthly growth rate was calculated from the length The study has been conducted in the Zanzibar frequency distribution. Channel (39° 10' 13" E, 6° 9' 4" S). The average Species composition - The bivalves temperature, salinity and oxygen concentration of the (LamelIibranchia: Anisomyaria) which grew on the waters during the study period are 27.6°C,34.21rooand floating cages belonged to families, Pteriidae (Pinetada 4.68 mIll respectively. The channel is calm during the vulgaris, P. margaritifera, and species of Pinetada southern monsoon (April to November) and rough and designated as A, B, and C), Anomiidae (Anomia turbid during the northern monsoon (November to laqueata), Pectinidae (Chlamys senatorious, March). The channel is flushed by the tide twice a day. Gloripallium pallium), and Ostreidae (Ostrea spp). Two plastic cages (0.5m3) were set in Sept. 1980, Three distinct members of the family Pteriidae could primarily for culturing rabbit fish Siganus eanalieul• not be identified to species level, which have been latus. After 103 days, a lot of marine organisms fouled described earlier by Spry2. The three species differ from the cages, the commonest being bivalves viz. Pinetada P. vulgaris and P. margaritifera in the following spp, Chlamys spp, Ostrea spp, Anomia spp and features-Pinetada sp. A: Left valve very inflated; Gloripallium spp. The relative abundance in numbers valves have radiating green bands. Pinetada sp. B: No

Table I-Relative Abundance of Bivalve Fouling Organisms on Floating Plastic Cages [Values represent percentage abundance]

2.220.32-1.18 Species (cages0.010.0213.174.4643.836.070.950.013.491.27(cage15.87Total24.8510.062.86(cage52.0726.9841.641693151.181.78 No.Inside8.88Both No. 1 No.4)& sidesettlement 1)4) Outside40.3647.325030.800.407.553.58- settlement UnidentifiedChlamysPinctada senatorious margaritiferasp. vulgaris bivalves B NumberPinctadaGloripa/liumOstrea ofsp. bivalves palliumCAsppAnomia laqueata (cage No.4) 283 INDIAN 1. MAR. seI., VOL. 11, SEPTEMBER 1982

:II 31 21 n 10tI' •• '"" R I.n 'l!••.10 a " . ~w,, . ,. 01: II II .6ro, '0 ~t~

" II, , I I

Fig. I-Length frequency distribution of (A) Ostrea spp, (B) Pinctada vulgaris and (C) Pinctada sp B. (Curves are drawn from running averages of 0.1 cm).

posterior ear, but dorsal margin is extended towards Pinetada sp. B (Fig. Ie). The results are comparable the posterior, purplish grey generally with radiating with those of P. vulgaris. Largest settlement occurs colourless bands. Pinctada sp. C: Long, narrow flaps at during the rainy period (between end of October and margins, creamy white with concentric bands. end of December, but mainly in November and early Relative abundance of various species recorded are December). given in Table 1.There were more individuals, of each Nine species of fouling bivalves were found on the species, growing on the outer than in the inner surface cages in the Zanzibar Channel, the most dominant of the cages. being Pinctada vulgaris and Ostrea spp. Cheah and Length frequency distribution and growth rate• Chua3 recorded the occurrence of bivalves viz. Length frequency distribution of Ostrea spp, Pinctada Pinetada vulgaris, Pinctada chemnitz, Pteria sp., Perna vulgaris and Pinctada sp. B is given in Fig. lA to C viridis, Modiolus sp., Chlamys transquebarous, Area respectively. auriculata, Barbatia siousis, Crassostrea sl'. and There are 8 peaks for Ostrea spp suggesting 8 age Marte$ia striata on the floating cages in the strait of groups. The 8 peaks also suggest that the oyster spat Penang. Although most ofthe genera reported by them settled every 2 weeks during the 103 days of study. were reported by Spry2 to occur in Tanzania waters, 1 Settlement was greatest during spring tides during only Pinctada and Chlamys were identified in this which more planktonic organisms in the water around study. The occurrence of similar species in Penang and Zanzibar harbour5 are reported. The peaks (Fig. lA) Tanzania, two areas which are far apart, clearly shows show that there was greater growth rate in younger wide distribution of these species in the tropics. individuals, and the growth rate decreased with Length frequency distribution indicates that the increasing age. These oysters grew at an average rate of frequency of spat settlement differs with the species. In 0.045 mm/day or 0.32 cm/week. Ostrea sp. settlement occurs every 2 weeks depending Unlike Ostrea spp, Pinctada vulgaris (Fig. lB) shows on the lunar and tidal cycles. This reflects the repro• 3peaks (2.0,2.5 and 3.3cm)each of which represents an ductive cycle of Ostrea sp.. Settlement of spat from age group. The first peak which is of smaller other oyster species could be monthly. Spawning ofthe individuals is the largest. This suggests that the spat oyster Crassostrea dentieulata in Sierra Leone takes which has grown to the individuals of the first peak place throughout the year, with some suppression settled at the time when the environmental conditions during heavy rains4. It may be that spawning of the were very favourable for settlement. Tanzania species is also suppressed during the long There are 4 peaks at 2.1, 2.5, 2.9, and 3.7 cm for rams.

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The authors record their sincere gratitude to the References International Foundation for Science (IFS), Sweden, 1 Bwathondi P 0 J, Unpublished data. for supporting this project through Grant No 263. 2 Spry J F, The sea shells of Dar es Salaam Part Il-Pelecypoda Thanks are due to Mr S M Mohammed for providing (Bivalves) in Tanganyika Notes and Records, 63 (1964) 1. 3 Cheah S H & Chua T E, Malay Nat J, 33 (1979) 39. hydrographic data and also to Mr A Mirza, D 4 Karnara A 8 & McNeill K 8, Preliminary oyster cuhure Mukaka, A Azzan, M Juma, K Hemmed, and I A experiments in Sierra Leone (Oce. Pap, Fisheries DivisioJ;l. Omar for their efforts in maintaining and mending the Min. Agric. and Nat. Res. Freetown, Sierra Leone) 1976. cages. 5 Wickstead J H, Proc Zool Soc Lond, 141 (1963) 577.

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